This invention relates to circuit boards on which are mounted various electrical and/or optical components, and particularly to electromagnetic interference (EMI) shields used with such components.
A common practice in the telecommunications industry and the like, where huge banks of electrical and/or optical components are used, is to mount numerous such components on printed wiring boards forming part of "circuit packs" and to mount the circuit packs in closely packed, side-by-side relation on grooved shelves within electrical cabinets. Each circuit pack printed wiring board (PWB) includes metal plated surface areas on which electrical and/or optical components are mounted and by means of which the components are electrically interconnected to one another and to various terminal connectors mounted, typically, at the front and rear ends of the circuit packs. The terminal connectors removably mate with connectors fixedly mounted on the cabinets whereby the various components on each circuit pack can be interconnected to other components within the electrical cabinet or, by means of interconnecting cables, to other components elsewhere in the telecommunication network.
Some of the electrical "components" (including combinations of devices and circuit elements) operate at quite high frequencies and, for shielding the components from being interfered with or interfering with other high frequency components, the respective components are disposed within shields mounted on the PWB and electrically grounded to grounded metal areas of the PWB.
One type of EMI shield, perhaps most closely related to the present invention, is known as a "casting" (from its method of fabrication) and, as shown in FIG. 1, comprises a generally flat plate including a number of ribs 10 or ledges extending from one surface of the plate. Typically, the casting is relatively large and a single casting encloses substantially all of one side of the PWB on which the components are mounted. All of the ribs are of the same height, and all the ribs engage a common surface of the PWB for forming (typically) a plurality of separate EMI shielded spaces or cavities for respective components on the PWB. For providing a tight fit of the ribs against the PWB surface, spaced apart openings 12 are provided directly vertically through the ribs for receipt (FIG. 2) of screws 14 for screwing the casting to the PWB. The openings 12 pass through enlarged area portions 16 of the ribs.
The pattern of cavity defining ribs (including a peripheral rib defining a continuous side wall of the casting) varies depending upon the design of the particular PWB with which each casing is to be used, and the castings tend to be relatively expensive owing to tooling costs. Also adding to the cost of the castings is that, for ease of fabrication and lightness of weight, the castings are of cast magnesium, a relatively expensive material.
While generally satisfactory for their purpose, although somewhat expensive, a problem arises with the use of such castings in situations where extremely high frequencies are involved. With such high frequencies, it is found that EMI leakage occurs through extremely small spaces between the casting ribs and the PWB surface caused by slight variations in the flatness of the PWB surface. A known solution for this problem is to coat the rib end surfaces with an elastic, electrically conductive silicone compound. The elastic silicone compound serves as an electrically conductive sealing gasket 18 (FIG. 2). Two problems exist. One is that the conductive silicone compound is provided by "in place" technology, i.e., it is beaded onto the ribs similarly as caulking is applied from a caulking gun. This is time consuming and expensive, and it is particularly difficult to apply the compound around the rib enlarged screw receiving areas 16. Secondly, to retain the beaded material in place, the material is cured. A problem with this is that the cured material adheres so well to the casting ribs that complete removal of the cured material is extremely difficult and practically impossible. An increasingly prevalent requirement for materials used in various industries is that the materials be recyclable. The magnesium castings, by themselves, are readily recyclable, but are not so when including cured conductive gaskets.
Another type of EMI shield extensively used with PWBs is a two piece shield (not illustrated) comprising a "fence" and a "lid". The fence is an annular member comprising a sheet metal wall enclosing an open space. In use, the bottom surface of the wall is engaged with a grounded, metal plated surface of the PWB by means of wall peg extensions which are press-fitted through corresponding openings through the PWB. The pegs are then soldered in place. The annular wall or "fence" encloses an area of the PWB surface on which components are mounted. A problem with the use of such fences is that the fences must be mounted in place prior to the mounting of the components for preventing damage to the components by the fence solder mounting process. Thus, the fence shield must be completed by a lid press-fitted over the fence wall and covering the component filled enclosed space.
Advantages of the two piece shield are that it is relatively inexpensive; the fence mounting and soldering processes are readily done by machine; and the press-fitted lid, while requiring a separate mounting operation, can be removed for access to the shielded components.
A problem with such two piece shield, however, is that no simple and readily applied means are presently known for adding fully effective elastic shielding means for electrically closing tiny spaces between the fence bottom surface and the PWB surface. Also, even with tight fits of the lids to the fences, tiny gaps invariably exist allowing passage of extremely high frequency EMI.
What is required, and provided by the present invention, is a simple and inexpensive shield which can be easily and removably mounted on a PWB and which includes elastic conductive means, i.e., an elastic gasket, for completely electrically engaging bottom surfaces of the shield with an irregularly flat surface of a PWB.